(1) Field of the Invention
The present invention relates to cordless telephone systems and, more particularly, to a scrambling and a descrambling circuit for a cordless telephone.
(2) Description of the Related Art
Cordless telephones are widely used because of the convenience. In many cordless telephone systems, the transmission signals between the cordless telephone handset and the base station that carry private conversations can be casually received and eavesdropped.
Scrambling technology is used to prevent eavesdropping of the private phone conversations developed in voice and non-voice communication systems.
Scrambling techniques used in voice communication systems are classified under four categories. These four scrambling categories are frequency domain scrambling, time domain scrambling, amplitude scrambling and two-dimensional scrambling. Details of these four scrambling skills are disclosed in the paper entitled "A Comparison of four Methods for Analog Speech Privacy"(IEEE Transactions on Communications. Vol. COM-29, NO. 1, January 1981.) by Jayant et al.
Among the four scrambling categories, frequency domain scrambling is classified as using the methods of frequency inversion, band-shift inversion and spread spectrum. The spread spectrum method changes the band width of a scrambled signal. The frequency inversion and band-shift inversion methods do not change the band width of a scrambled signal.
In cordless telephone systems, the band width of an audio signal is generally restricted to a frequency of 300 Hz to 3400 Hz. Because the spread spectrum method changes the band width of the scrambled signal, it cannot be used for scrambling the restricted frequency band in cordless telephone systems.
Referring to the attached drawings, prior scrambling skills according to the frequency inversion and band-shift inversion methods will be described below.
The circuit in FIG. 4 is a frequency inversion scrambling circuit implemented in a transmitter such as located in a cordless telephone base unit. The circuit in FIG. 4 can also be used in descrambling circuits of a receiver such as located in a cordless telephone hand set. The frequency inversion scrambling circuit includes a first low pass filter 41 for receiving an audio input signal IN, a modulator 43 for receiving a predetermined split frequency Fs and the output signal of the first low pass filter 41. A second low pass filter 42 receives the output signal of the modulator 43 and generates an output signal OUT.
The first low pass filter 41 passes through the input signal IN for a frequency band lower than a given cutoff frequency. The modulator 43 outputs two frequency components, which are inverted and non-inverted signals of the output signal from the low pass filter 41. The modulator 43 transposes an inverted frequency component and a non-inverted frequency component symmetrically on the left and right side of the split frequency, respectively. The second low pass filter 42 passes through the output signal, from the modulator 43, having a frequency band lower than a given cutoff frequency, and provides the filtered signal that serves as an output signal OUT.
FIGS. 5A-5C and FIGS. 6A-6C illustrate the scrambling and descrambling waveforms respectively in the case that the cutoff frequency of the first and second low pass filter 41 and 42 is 3.4 KHz and the split frequency Fs of the modulator 43 is 3.7 KHz.
FIG. 5A shows a filtered output signal, from the first low pass filter 41 for an audio signal having a frequency band lower than 3.4 KHz. FIG. 5B shows the output signal of the modulator 43. The filtered output signal from the low pass filter 41 is inverted and the inverted signal is symmetrical on the left and right side of the split frequency Fs at 3.7 KHz. FIG. 5C shows the output signal OUT of the second low pass filter 42. The output signal OUT has a frequency band lower than the 3.4 KHz and is obtained low pass filter 42 of filtering the output signal of the modulator 43.
From the comparison of FIGS. 5A and 5C, the scrambling circuit inverts the low and high frequency components of the input signal IN, such that the low and high frequency components of the input signal IN are transformed into the high and low frequency components, respectively, of the output signal OUT.
The scrambled audio signal, from the scrambling circuit, is transmitted to a receiver by a communication channel. The transmitted signal is descrambled by the descrambling circuit in the receiver to obtain the original audio signal.
FIGS. 6A-6C show the descrambling procedure, when the circuit in FIG. 4 is used as a descrambling circuit in the receiver. The scrambling and descrambling circuitry are essentially the same in configuration. For convenience of description, we also consider the circuit in FIG. 4 as a descrambling circuit.
FIG. 6A shows the output signal of a first low pass filter 41. The first low pass filter 41 accepts a transmitted audio signal and passes through the transmitted audio signal having a frequency band lower than a given cutoff frequency. FIG. 6B shows the output signal of the modulator 43. The modulator 43 outputs two frequency components, which are an inverted and non-inverted signals of the output signal from the low pass filter 41. The modulator 43 transposes an inverted frequency component and a non-inverted frequency component symmetrically on the left and right side of the split frequency, respectively.
FIG. 6C shows the output signal of the second low pass filter 42. The second low pass filter 42 accepts the output signal of the modulator 43 and passes through the signal having a frequency band lower than a given cutoff frequency. The second low pass signal provides the filtered signal that serves as an output signal OUT. The cutoff frequencies of the two low pass filters and the split frequency of the modulator in the receiver are the same as those in the transmitter. As shown in FIGS. 5A and 6C, the input signal IN of the scrambling circuit in the transmitter is the same as the output signal OUT of the second low pass filter in the receiver.
FIG. 7A is a waveform showing the input signal of the modulator 43, and FIG. 7B is a waveform showing the output signal of the modulator 43.
The scrambling and descrambling circuits shown in FIG. 4 are widely used since the configuration is very simple. However, the scrambled signal output by the circuit described above is easily descrambled with a simple RF (radio frequency) receiver. Thus, eavesdropping of conversations from the cordless telephone can be easily conducted by tuning into the split frequency of the telephone modulator. Moreover, even when the exact split frequency is unknown, eavesdropping can still be performed as long as the RF receiver is tuned within the vicinity of the split frequency. For example, when the split frequency is 3.7 KHz, setting the modulator of the RF receiver to a split frequency of 3.5 KHz, still allows eavesdropping of the conversation.
FIGS. 8A and 8B show a scrambling procedure according to the band-shift inversion method. FIG. 8A is a graph showing power density of an audio signal as a function of frequency before scrambling. FIG. 8B is a graph showing power density of the audio signal as a function of frequency after scrambling. As shown in FIGS. 8A and 8B, an audio signal is separated into two parts, a low frequency band and a high frequency band, by a band split frequency Fs. The form of FIG. 8B is an inverted form of FIG. 8A.
The scrambling circuit according to the prior band-shift inversion method can obtain a relatively high security level. However, the configuration of the scrambling circuit is quite complicated. However, the scrambling circuit according to the band-shift inversion method also still allows room for a conversation to be eavesdropped by someone, because it is easy to determine the band split frequency Fs.